The present invention relates to a calibration device for extruded sections, in particular for hollow sections, pipes and alveolar plates of any geometrical structure and made of any extrudable thermoplastic material.
In systems for extrusion of plastic materials, molten extruded plastic material coming from an extrusion die head and having a closed-cell geometrical shape is made to pass through a calibration assembly which enables the surfaces of the extruded section to be cooled off, at the same time maintaining the geometrical shape impressed by the die head.
The calibration assemblies according to the prior art are provided with calibration plates as illustrated in the cross-sectional view of FIG. 1 and designated by the reference number 100. FIG. 1 shows two calibration plates 100 arranged at an appropriate distance apart so as to leave a gap 101 through which the extruded section can pass.
Each plate 100 is provided with ducts 102 for the circulation of a coolant and ducts 103 for vacuum. The ducts 103 for vacuum have radial slits 104 that set them in communication with the gap 101 through which the extruded section is to pass. The cooling ducts 102 are connected to a cooling system, and the vacuum ducts 103 are connected to a vacuum-forming system.
In order to enable cooling and maintenance of the geometrical shape of the extruded section, the calibration plates 100 are provided with a cooling circuit, made up of the ducts 102, to which a refrigerating unit may be connected in the case where an alveolar section of plastic material of the PP-HDPE type is to be produced, or else a thermoregulation control unit may be connected in the case where the extrusion material employed is PC-PMMA.
In order to cause the surfaces of the extruded section to adhere or closely fit to the calibration plates 100 and then perform calibration of the extruded section, vacuum pumps are used which are connected to the vacuum ducts 103, in such a way as to create a vacuum in the slits 104 and hence in the gap 101, thus causing the surface of the extruded section to adhere to the plates 100.
However, the aforesaid calibration devices according to the prior art have a number of drawbacks.
In fact, the adhesion of the extruded section to the surfaces of the calibration plates 100 due to vacuum inevitably creates a considerable friction between the extruded section and the plates. This impairs the ease of slip of the extruded section, and consequently reduces the sliding speed of the extruded section, and hence the out put capacity of the system or plant.
In addition, the aforesaid high-friction sliding of the extruded section on the calibration plates spoils the surface quality of the product and decreases its optical characteristics. For this reason, with the prior art calibration devices it is not possible to obtain perfectly smooth surfaces having optimal optical characteristics.
An object of the present invention is to eliminate the drawbacks of the prior art by providing a calibration device for flat extruded sections that enables high production rates to be achieved and at the same time ensures an excellent surface quality of the extruded section.
The calibration device according to the invention comprises two flat calibration plates arranged parallel to one another and at a distance apart such as to form a gap through which a flat extruded section can pass. Each calibration plate comprises a cooling circuit for cooling the plate and a vacuum circuit for creating a vacuum in the gap in which the extruded section is to pass.
The peculiar feature of the calibration device according to the invention is that the said device is provided with air-intake means for introducing air into the gap between the calibration plates in such a way as to form air cushions or pads between the flat surfaces of the extruded section and the flat surfaces of the calibration plates.
The said air cushions tend to reduce the sliding friction between the surfaces of the section that advances through the gap and the surfaces of the calibration plates. In this way, the section can advance at a higher speed than in the case where no air cushions are provided, with the result that a higher production rate is achieved.
In addition, the aforesaid reduction in the sliding friction between the surfaces of the section and those of the plates guarantees a lower degree of deterioration of the surfaces of the section, with the result that products with a better surface finish, and hence a better optical quality, are obtained.
In addition, the air introduced into the gap of the calibration device is sucked in by means of the vacuum circuit. Consequently, there is a continuous circulation of fresh, cool air which comes into contact with the surfaces of the section, thus improving the cooling characteristics of the calibration device according to the invention.